Fuel driven turbine based power generators and plants have been in existence for many years. They generally comprise a means of burning a mixture of air and fuel vapor within a combustion chamber to drive an expansion turbine which in turn drives an electrical generator that produces electricity. An upstream compressor turbine is typically used to draw in ambient air and increase its pressure prior to being introduced into the combustion chamber, wherein the pressurized ambient air is combined with high pressure fuel vapor and ignited in the combustion chamber to drive an expansion turbine which in turn drives an electric generator to produce electricity.
Another common feature of conventional fuel driven turbine generators is that the compressor turbine, expansion turbine and electric generator are typically located on the same shaft so that if one rotates the others rotate as well. In such case, initiating the operation of the system can be accomplished by rotating the compressor turbine which in turn causes the expansion turbine and generator to rotate as well. That is, as the compressor turbine begins to rotate and causes the ambient air to be compressed and introduced into the combustion chamber, the energy created as the fuel vapor is burned within the combustion chamber can help drive the expansion turbine which in turn continues to drive the compressor turbine. Because the compressor turbine, expansion turbine, and electrical generator are located on the same shaft, by using the energy produced by the combustion chamber to drive the expansion turbine, all three components can be driven until a steady state condition of operation is achieved.
As a result of this configuration, however, a downside is that nearly two thirds of the energy produced by the system is typically used to drive the compressor turbine, such that only about one third of the total energy output is available to drive the electrical generator to produce electricity. That is, a majority of the work performed by the system is essentially recycled and reused to drive the compressor turbine, wherein less work becomes available to drive the electrical generator, which makes the system inefficient.
Another drawback to this type of system is that when the ambient temperature of the inlet air increases, the air becomes less dense, and therefore, more energy is needed to drive the compressor turbine—to compress the same amount of air and perform the same amount of work. And when the amount of work needed to drive the compressor turbine is increased, this additional energy is taken away from the output, wherein less energy becomes available to drive the electrical generator, which in turn, results in less energy being produced for the same amount of fuel that is consumed.
For this reason, many attempts have been made in the past to reduce the temperature of the inlet air introduced into the compressor turbine which has the effect of increasing the density of the air and the efficiency of the fuel driven turbine generator. At lower intake temperatures, air drawn by the compressor turbine is denser and therefore has greater mass flow and thus less energy is required to enable the compressor turbine to perform its work, and, as a result, more energy becomes available to drive the electrical generator and generate electricity. That is, a greater portion of the energy produced by the system can be used to drive the electrical generator and thereby produce electricity. Note that the compressor turbine consumes energy on the basis of the volumetric flow that it pressurizes, so when the cold air is denser, it passes on more compressed air mass to the combustion chamber for the same amount of expended work or compressor turbine energy.
For these reasons, various technologies have been devised and developed in the past to cool the inlet air before it is introduced into the compressor turbine. The most common commercially available coolers are: 1) evaporative coolers, 2) fogging coolers, 3) high pressure fogging coolers, 4) wet compression coolers, 5) mechanical chillers, 6) absorption chillers, and 7) liquid air coolers.
The downside to these cooling systems is that many of them have specific disadvantages that make them ill suited for widespread application in connection with this technology. For example, fogging and evaporative coolers typically only bring the inlet air temperature down from the dry bulb temperature (Tdb) to the wet bulb temperature (Tw), and at 100% relative humidity, these values can equal each other (Tdb=Tw), wherein no cooling is then possible. Even if the relative humidity is lower, such as down to about 40%, the maximum temperature drop is still only Tdb−Tw, or about 15 to 25 degrees F. In such case, if the ambient air temperature is relatively high, such as 100 degrees F. or higher, the lowest temperature that can be achieved may be about 75 degrees to 85 degrees F. For these reasons, despite their lower installation costs, fogging coolers, evaporative coolers and wet compression coolers are typically ill suited for use in many applications, including those in various areas of the world such as in hot humid climate regions.
Mechanical and absorptive coolers are also expensive to install and operate, and have a high cost of maintenance, wherein these costs can significantly reduce the cost savings that could otherwise be achieved by reducing the inlet air temperature to increase efficiencies of the system. Another disadvantage of these conventional cooling systems is that they typically use chemical refrigerants such as ammonia, lithium bromide and Freon which can represent an environmental hazard. It should be noted that each chilling system may have a different fluid in the primary heat exchanger coil, as well as in the secondary heat exchanger coil, and different compressors that create different initial costs, maintenance costs and environmentally permitted fluid disposal costs. The lithium bromide system also involves the maintenance of a vacuum.
Another significant disadvantage is that these past cooling systems when used in connection with conventional fuel driven turbine generators typically cannot be used to reduce the inlet air temperature to below freezing. This is because when there is any moisture in the ambient air, ice particles can form that can damage the turbine blades and reduce the efficiency of the system. Typically, the entrance into the combustion chamber of the gas turbine set is in the shape of a nozzle or nacelle and the compressor turbine blades are located a short distance downstream, wherein the inlet cross-sectional area of the nacelle is often larger than the downstream cross sectional area of the nozzle, so that there is an isentropic acceleration of the air speed along with an associated temperature decrease. Accordingly, an associated problem that can occur when reducing the temperature of the inlet air in this manner is that the constriction of the air flow path inside the nacelle of the compressor turbine can cause the air inside to become overcooled, i.e., colder than it should be. Although lower inlet air temperatures can result in greater system efficiencies, as discussed above, the downside is that if the temperature of the inlet air is reduced below freezing, ice particles can begin to form, wherein not only can the ice particles lead to a reduction in power output, but in many cases, they can strike the high speed turbine blades as the air is being accelerated into the gas turbine which can cause pitting and damage thereto.
In this respect, it has been established mathematically and empirically that the drawn-in air can be cooled by up to about 9 to 10 degrees F. or more during acceleration of the inlet air into the compressor turbine, which is based on an air velocity of up to 300 feet per second or more traveling through the nacelle. In such case, even if the initial inlet air temperature is carefully set to a few degrees above freezing, when the air accelerates through the nacelle and becomes colder, ice particles can begin to form that can damage the turbine blades. Accordingly, in order to avoid damage to the turbine blades, and maintain system efficiency, the initial inlet air into the compressor turbine typically must be higher than about 42 degrees F., especially whenever there is any moisture in the air. The result has been that the system was limited to having the inlet air temperature reduced to only a minimum of about 42 degrees F., which limited the extent to which the system efficiencies could be improved.
In order to avoid the formation of ice particles within the intake air of the fuel driven turbine generator, it has been a normal practice to warm the intake air before entering into the compressor turbine, notwithstanding that there has been an effort to reduce the temperature thereof. In such case, additional heating or anti-icing devices have been employed, such as, for example, a system that feeds re-circulated hot compressor air into the intake air using a steam-heated heat exchanger or even using an electric heating system. The down-side to using such systems is that additional energy is consumed to power the heating system. Moreover, by increasing the temperature of the inlet air, notwithstanding the desire to reduce the temperature thereof, additional inefficiencies are introduced thereby.
Because of the above disadvantages, another associated problem with existing cooling systems of this kind is that each one is limited in its operating regime so that a single system is not normally suitable for widespread application. For example, fogging, high pressure fogging, wet compression fogging and evaporative coolers are typically not useful in relatively hot humid climates because they can't reduce the temperature of the inlet air low enough. Likewise, mechanical and absorptive chillers may be too expensive to install and operate and can produce environmental concerns that may not justify the improved efficiencies they provide. And, most importantly, past cooling systems suffer from being limited in their ability to reduce the temperature of the inlet air to below about 42 degrees F., such as when there is any moisture in the air, which is almost all of the time.
Accordingly, what is needed is an alternative system capable of increasing the efficiencies of a fuel driven turbine generator set without causing additional system inefficiencies to occur.